The most constant complaint of damage shields is that they can be stacked on top of each other. That if anything should be the point that any change tries to address but adding a cast time not only doesn't directly deal with that issue it makes damage shields not worth it for the usefulness they provide as they are still the same size as they were before except now you can't actively defend yourself with them
Frankly this change is overkill it's too much of a nerf the critical damage to them is a good and necessary change but cast time is too much i cannot see how magicka sorcs will be able to compensate for such a heavy hit on their defensive ability.
Please don't do this. I see no way I could play a pvp magicka sorcerer if this change goes live.